1. Field of the Invention
The present invention relates in general to the field of information handling system displays, and more particularly to a system and method for sequential driving of information handling system display backlight LED strings.
2. Description of the Related Art
As the value and use of information continues to increase, individuals and businesses seek additional ways to process and store information. One option available to users is information handling systems. An information handling system generally processes, compiles, stores, and/or communicates information or data for business, personal, or other purposes thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems.
Portable information handling systems have achieved performance levels which have encouraged end users to purchase portable systems as replacements for desktop systems. The higher performance levels offered by portable information handling systems generally result in greater power consumption, which reduces the operating time for battery operations. Portable information handling system displays, typically liquid crystal displays (LCDs), are often a significant consumer of battery power. LCDs present images by illuminating a backlight through a panel of liquid crystal pixels. The color of light that passes through the liquid crystals is altered by filters and the crystal state. Conventional LCDs use cold cathode fluorescent light (CCFL) to generate the backlight. Although CCFLs are generally energy efficient, recent improvements have made LEDs an attractive alternative to CCFLs as LCD backlights because LEDs are more energy efficient and operate on direct current. LEDs are available that produce white light (WLEDs) and that produce red, green and blue light (RGB LEDs) which combine to provide white light.
LED backlights are typically driven under the management of an integrated circuit (IC). Typically, the LEDs are arranged in strings with a backlight consisting of between four and six strings and each string having approximately ten LEDs. Although LEDs are powered by direct current, driver ICs generally use pulse width modulation (PWM) brightness dimming instead of analog dimming due to non-uniformity issues and color shift associated with analog dimming. However, when PWM dimming is implemented, high inrush currents and transients sometimes result from all of the LEDs turning on and off simultaneously. The inrush currents and transients can cause issues with electromagnetic interference (EMI) and acoustic noise and tend to require relatively high input and output capacitance be used for the LED driver. One solution is to sequentially drive the LED strings over a PWM time period so that the inrush currents and transients occur one string at a time and thus have a reduced impact. For example, some LED backlight drivers use a fixed time delay of a portion of the PWM period for the application of power at each LED string, such as ⅛th of the PWM period. Other LED backlight drivers use a fixed time delay of a set amount of time, such as 20 ns. When the amount of delay between sequential dimming of LED strings is fixed, the ripple current varies with the duty cycle depending upon the number of strings used in the backlight.